Single and twin screw extruders are well known continuous processing apparatus which are mainly used in the polymer and ceramics but as well in food industry where e.g. pasta and snack products are produced. Since 1992 (DE 4202231 C1) extruders were also suggested to be used for continuous freezing of frozen deserts like ice cream.
Processing Aspects
As described in several publications (see literature review 2-19) a low temperature extruder allows for deep-freezing of ice cream and other food masses like yoghurt and fruit pulps up to a high degree of frozen water fraction (80-90% related to the freezable water fraction) under simultaneously acting mechanical stresses by shear flow.
The dissipated heat caused by viscous friction in the highly viscous partially frozen systems (dynamic viscosity up to 104 Pas) has to be transferred in addition to the crystallization heat (freezing) efficiently, whereas an equilibrium between generated and transferred heat is adjusting dependant on the heat transfer coefficient k (describes the heat transfer through a product layer adhering to the inner wall of the extruder housing to and through this steel wall and into an evaporating refrigerant contacting the outer wall of the extruder barrel.
Up to now, maximum heat transfer coefficients are reached by a proper choice of extrusion screw geometries with a narrow leakage gap between the extruder barrel flight tip and the inner wall of the extruder barrel in order to efficiently replace the frozen material layer next to the extruder barrel wall, and by use of an evaporating refrigerant (e.g. ammonia) for cooling of the extruder housing. The shear rates generated in the screw channel are narrowly distributed due to the use of screw geometries with low, constant screw channel height and a slight axial shift of the screw arrangement within twin-screw extruder systems (EP 0561 118B1). This means, that there are no expanded zones with either very high or very low shear rates. At maximum shear rates of approx. 20−30 s−1 for typical ice cream masses outlet temperatures of −12 to −18° C. at the extruder outlet are reached.
The minimum draw temperature of the mass at the extruder outlet depends on the freezing point depressing properties of the mass and the related viscosity of the mass at respective temperature as well as on the mechanical energy dissipation caused by the viscous friction.
In ice cream mass extrusion (e.g. according to patents EP 0561118, U.S. Pat. No. 5,345,781), only a small pressure gradient over the extruder length is generated. The total pressure difference between extruder in- and outlet is in general ≦1-5 bars. This guarantees the avoidance of de-mixing the gas liquid (foam) mixture, which is still rather low viscous at the extruder inlet, to a large extent. The specific extruder screw configuration as well as the screw arrangement (twin screw) in the low temperature extruder according to EP 0561118 or U.S. Pat. No. 5,345,781; DE 4202231C1 respectively in addition apply a gentle, efficient mixing of the mass. This is particularly achieved by an appropriate flow stream distribution in the screw flight overlapping/intermeshing zone between the screws in the twin screw arrangement.
Product Aspects
Beside beforehand described apparatus and process related aspects there is main interest in the product specific advantages properties which can be achieved within ice cream treated by low temperature extrusion. Generally it can be stated that such advantageous properties generated by low temperature extrusion relate to a more finely dispersing of the micro-structural ice cream components: ice crystals, air bubbles/air cells and fat globule agglomerates. The extent of such dispersing effects also depends on the ice cream recipe. The following description relates to typical standard recipes of vanilla ice cream, however with variations in the contents of fat/milk fat (0-16%) and in dry matter (35-43%). The advantageous special properties achieved for low temperature extruded ice creams are related to the main structuring disperse elements in ice cream being the water ice crystals (1) the air bubbles/air cells (2) and the fat globule agglomerates (3) which are all much more finely dispersed under the high mechanical stresses acting in laminar shear and elongation flow fields within the extruder flow under low temperature conditions.
For ice crystals, secondary nucleation effects by crystal attrition and crystal breakage in addition to further primary ice crystal nucleation at the inner barrel wall, nucleation lead to size reduction by a factor 2-3 compared to conventional ice cream processing in freezer and subsequent hardening tunnel. Mean air bubble/air cell size is reduced by a factor 3-5 compared to the conventional process due to increased acting shear stresses leading to bubble/air cell break-up.
The intensity of the mechanical treatment in the extruder flow strongly depends on mass viscosity, which is related to the frozen water fraction at a specific temperature. Over the cross section of the extruder screw channel, which forms a narrow annular gap the shear stresses are rather homogeneously and narrowly distributed (now flow zones with stress peaks). Over the extruder length, the mechanical energy input increases with increasing residence time of the ice cream in the extruder channel as well as with the increase of the mass viscosity as a result of an increasing frozen water fraction.
A local destruction of the ice cream structure by too high energy dissipation and related friction heat generation, is avoided at process/apparatus shear rates typically applied (EP 056118).
In fat containing ice creams there are fat globules with atypical main size of approximately 1 micron and below in globule diameter as a result of the ice mix treatment in the liquid state within high pressures homogenizers. Such fat globules also experience an increased mechanical treatment in the low temperature extrusion process. For the fat globules this treatment leads to de-hulling of the fat globule surface from protein/emulsifier membranes and partially also to a strong deformation of the fat globules by the intensive shear acting in the extruder. As a consequence, such treated fat globules are expected to have stronger hydrophobic interactions. Consequently, there is also an increased affinity to the gas/air bubble interface. The increased interaction between treated fat globules leads to the formation of fat globule aggregates. However, the movability of such fat globules in the highly viscous low temperature loaded ice cream is low and consequently there is no chance for the formation of largely expended fat globule aggregates reaching a sensorially (mouth) detectable size. This avoids the generation of a buttery mouth feel causing structure.
From the sensorial view point, the smaller ice crystals and gas/air bubbles as well as the mechanically treated but not too largely agglomerated fat globules lead to a strongly increased perceptible creaminess of the product. At the same time, other sensorial attributes are also significantly positively influenced by low temperature extrusion of the ice cream like the melting behavior, the coldness sensation in the mouth and the scoop ability.
Due to the increased fine dispersity of the disperse ice cream components causing the beforehand described increase of creaminess sensation, low temperature extrusion allows to generate comparable creaminess like conventional ice cream processing at much lower fat content.
Construction Aspects (Extruder Screw(s))
To generate a homogeneous microstructure of the ice cream (1) and at the same time reach very low extruder outlet mass temperatures of lower than ca. −12° C. (2) (standard vanilla ice cream) the construction of the extruder screw(s) with respect to the related flow conditions at adapted rotational speed are of crucial importance.
EP 561118 describes a twin screw extruder for continuous freeze-structuring of ice cream using screw geometries, with especially flat screw channels (ratio channel height H to channel width W about 0.1, ratio of channel height to outer screw diameter about 0.1) and a screw angle of ca. 22 to 30°.
EP 713650 relates to a process which also includes a twin screw extruder for the extrusion of frozen products. The screw characteristics are only described by the ratio of extruder length to screw diameter.
EP 0808577 describes a comparable process using a single screw extruder with similar construction principles of the screw like given in EP 713650.
WO97/26800 claims process and apparatus for the manufacture of frozen eatable foams like ice cream using also a single screw extruder. Characteristic properties for the geometry of the extruder screw are the ratios: length of the screw to inner diameter of the extruder housing between 5 and 10, ascending height of the screw to the screw outer diameter between 1 and 2 as well as outer diameter of the screw to inner screw diameter between 1.1 and 1.4. The extrusion screw has only 1 screw flight.
There are also low temperature extruders known (single and twin screw extruders) for the treatment of ice cream with 2-6 screw flights, preferably 2-5, and a screw angle of 28 to 45° preferably 32 to 45°. Preference is given to a ratio of general height to general width of smaller than 0.2 but larger than 0.1. Preferred ratio of screw channel length to inner screw diameter is fixed to 2 to 10, preferably 2-4. This leads to rather short extruders.
The basic difficulty in continuous freeze structuring of ice cream within low temperature extrusion systems relates to the combination of a mechanical treatment and the simultaneous solidification by ongoing freezing. The latter leads to the increase of viscous friction based energy dissipation proportional to the viscosity and consequently to the need of transferring this dissipated energy in addition to the crystallization enthalpy set free by the freezing process. This coupled heat transfer is limited by the rather low heat conductivity of the foamed ice cream mass and the related achievable heat transfer coefficient k in the laminar low temperature extrusion flow of the ice cream. The heat has to be transferred from the flowing ice cream mass through a non-mixed inner barrel wall adhering ice cream layer, through the barrel wall and to the refrigerant contacting the outer barrel wall. The optimization of the flow conditions in the extruder with respect to maximally improved product properties, aims the maximum shear treatment to reach most finely dispersed microstructure at minimum extruder outlet temperature.
In the extruder screw geometries, conventionally described for low temperature extrusion processing a high mechanical treatment efficient for micro-structuring is only reached in the end zone of the low temperature extruder close to the extruder outlet. The length of this structuring-efficient end zone reaches in general less than 50% of the total extruder length.
Due to the fact that, in general ice cream pre-frozen in a conventional ice cream freezer, is transferred into the low temperature extruder at inlet conditions of −5° C. and approximately 35 to 45% of freezable water fraction frozen, this mass experiences only low shear stresses in the extruder entrance zone up to about 50% of the extruder length. The treatment in this extruder domain does not contribute to finer dispersing of the microstructure components (ice crystals, air bubbles/air cells, fat globule agglomerates)
Like shown in recent research work there is even an increase in air bubble/air cell size detected in the first 30 to 50% of the extruder length. The reason for this is the shift in the dynamic equilibrium between air bubble dispersing and air bubble coalescence towards increased contribution of the coalescence due to the lower acting mechanical stresses compared to the precedent treatment of the ice cream in the conventional freezer.
FIG. 1 shows exemplarily such an effect of the air bubble size development along the extruder length in the first 150 mm of a pilot extruder screw channel (15% of the extruder length). In this domain the mean bubble diameter is increased by approx. 25% (see also FIG. 2). Only after 400 to 450 mm (≈40-45% of total length 1000 mm, 65 mm outer extrusion screw diameter and 7 mm screw channel height), the efficient fine-dispersing starts.
Experiments with various screw geometries have confirmed that a viscosity-adapted increase in shear treatment in the first 25 to 70% of the extruder channel length allows to improve this situation remarkably up to negligible coarsening of the structure in the inlet zone, thus allowing for a much better use of the extruder volume.
Problem
The problem of the invention is to freeze food masses continuously to highest possible frozen water fractions of larger than 60 to 65% of the freezable water fraction under simultaneous mechanically induced micro-structuring of the disperse components like ice crystals, air bubbles/air cells and fat globules/fat globule aggregates down to characteristic mean diameters below about 10 microns and narrow diameter distributions (x90,3/x10,3≦10).
A further problem is to provide a device to carry out such a process.
Advantages
With the inventive process, ice cream masses can be continuously deeply frozen and similarly optimally micro-structured at minimized energy-/power input not possible before. This is enabled by optimized heat transfer conditions from the ice cream mass to the evaporating refrigerant, up to high frozen mass fractions of 80-90% of the freezable water fraction and very low related temperatures at the outlet of the inventive low temperature extrusion process of −12 to −18° C.
The microstructure of this-like treated frozen masses leads to advantageous rheology which provides very good forming, shaping, portioning and scooping properties at much lower temperatures than known before.
Furthermore all low temperature extruded ice cream masses can be packaged and stored without intensive additional hardening (deep cooling), making conventional high energy consuming hardening tunnels no longer necessary.
Another advantage relates to the possible reduction of the fraction of expensive ingredients, conventionally used (e.g. milk fat, emulsifiers) for optimizing consumer relevant properties like creaminess necessary in conventionally processed ice cream.
Ice cream, which is optimized according to this patent application, shows improved creaminess at much lower milk fat content (reduction 3-6%) and without the need of emulsifiers. The reduction fat is of particular nutritional interest.
Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures.